Liver cancer is one of the malignant tumors that can lead to death. Its malignant degree is high, in China, the mortality rate of liver cancer is the second highest, only less than the mortality rate of gastric cancer. According to statistics, there are about 300,000 people with newly developed liver cancer each year in China, and about 110,000 people die each year because of this disease. Hepatitis B Virus (HBV) infection may lead to hepatitis, hepatic cirrhosis and primary liver cancer. In China, more than 80% of liver cancer patient had their liver cancer developed after HBV infection, which is also referred to as post-hepatitis B liver cancer.
The HBV is a DNA virus having a length of about 3.2 Kb, and contains overlapping open reading frames (ORFs) that are responsible for transcription and expression of the hepatitis B virus surface antigen (HBsAg), the hepatitis B virus core antigen (HBcAg), the hepatitis B virus polymerase, and the hepatitis B virus X antigen (HBxAg) (also known as hepatitis B virus X protein, HBx), wherein HBx is an indispensable factor in HBV DNA replication. Because HBx is an indispensable factor in HBV DNA replication, to inhibit the function of HBx may lead to suppression of HBV infection, as well as the subsequent hepatitis and hepatic cirrhosis.
In addition, as a trans-activator factor, HBx can promote the growth and proliferation of liver cancer, thus is also known as oncoprotein. Studies at molecular level, cellular level and animal level have shown that HBx has strong effect in promoting proliferation and migration of liver cancer cells. Transgenic mouse experiments also showed that HBx can cause liver cancers. The results of numerous studies further indicated that persistence of HBV infection could lead to chronic liver diseases, including chronic hepatitis, repeated chronic hepatitis, which subsequently causes hyperplasia of fibrous connective tissue in liver tissues and hepatic cirrhosis, and most of liver cancer based on hepatic cirrhosis. It has been well established that HBx plays a critical role in the development and progress of chronic liver diseases (including hepatitis, hepatic cirrhosis and liver cancer). Consequently, HBx becomes an important target for preventing and treating liver diseases.
Currently, the treatment of liver cancer is mainly through surgery, supplemented with interventional therapies, whereas chemotherapy is usually not very effective. Clinical data show that the rate of finding HBsAg and HBxAg positively expressed in liver cancer tissues is more than 80% or even 90%. Since HBx is an important pathogenic factor in the occurrence and development of liver cancer, identifying its specific inhibitors would have great theoretical and clinical significance. However, due to incompleteness of three-dimensional conformation analysis of HBx, it is rather difficult to design its chemical inhibitors through HBx's three-dimensional conformation information.
Fragmental polypeptides can be used as medicine, and they have already been widely used in clinical practices. For example, thymopeptide is a thymopentin extracted from calf thymus with the function of promoting lymphocyte transformation, enhancing phagocytic activity of macrophages, and can be used to treat a variety of immunodeficiency diseases. The characteristics of the polypeptide drugs include well-defined pharmacokinetics effects, safety, and easy to be manufactured. However, because polypeptide drugs tend to be affected by protease in vivo and have low stability and short half-life, the efficacy thereof is usually not ideal.
Amino acids can exist in the configuration of levorotation (L-amino acids) and dextrorotation (D-amino acids). All natural amino acids in human or animal body are L-type without any D-amino acids. Proteolytic enzymes for D-amino acids do not exist in organisms. Therefore, if the L-type natural amino acids are substituted with corresponding D-types without affecting the amino acid sequence when preparing polypeptide drugs, stability of the polypeptide drugs in blood can also be improved. However, how to improve degradation resistance of polypeptide drugs while not reducing its binding characteristics so as to maintain the pharmaceutical efficacy remains an important issue in polypeptide drug development.
The inventors of the present invention have discovered a polypeptide with a function of inhibiting hepatitis B virus X protein and solely comprised of natural L-amino acids (please refer to Chinese invention patent no. ZL201110061840.5 for details, the disclosure thereof is hereby incorporated by reference in its entirety). The polypeptide comprises the amino acid sequence as shown in SEQ ID NO:12. The polypeptide has a function of inhibiting HBx activities at molecular level, cellular level and animal level and can be used in the treatment or prevention of hepatitis, hepatic cirrhosis and liver cancer caused by hepatitis B virus infection.